Method for operating fuel cells on impure fuels

ABSTRACT

The electrocatalysts in certain fuel cell systems can be poisoned by impurities in the fuel stream directed to the fuel cell anodes. Introducing a variable concentration of oxygen into the impure fuel stream supplied to the fuel cells can reduce or prevent poisoning without excessive use of oxygen. The variation may be controlled based on the voltage of a carbon monoxide sensitive sensor cell incorporated in the system. Further, the variation in oxygen concentration may be periodic or pulsed. A variable air bleed method is particularly suitable for use in solid polymer fuel cell systems operating on fuel streams containing carbon monoxide.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a division of U.S. patent application Ser. No.09/221,206 filed Dec. 23, 1998, now U.S. Pat. No. 6,210,820 B1, whichis, in turn, related to and claims priority from U.S. Provisional PatentApplication Ser. No. 60/091,531 filed Jul. 2, 1998, entitled “SensorCell For An Electrochemical Fuel Cell Stack”. The '206 non-provisionaland '531 provisional applications are each incorporated herein byreference in their entirety.

FIELD OF THE INVENTION

The present invention relates to methods for operating fuel cells onimpure fuels. In particular, it relates to methods for substantiallyreducing the effect of or preventing carbon monoxide poisoning of fuelcell anode electrocatalysts by introduction of a variable concentrationof oxygen into the fuel stream.

BACKGROUND OF THE INVENTION

Electrochemical fuel cells convert reactants, namely fuel and oxidant,to generate electric power and reaction products. Electrochemical fuelcells generally employ an electrolyte disposed between two electrodes,namely a cathode and an anode. An electrocatalyst typically induces thedesired electrochemical reactions at the electrodes. In addition toelectrocatalyst, the electrodes may also comprise a porous electricallyconductive sheet material, or “electrode substrate”, upon which theelectrocatalyst is deposited. The electrocatalyst may be a metal black,an alloy or a supported metal catalyst, for example, platinum on carbon.

Solid polymer electrolyte fuel cells employ a membrane electrodeassembly (“MEA”) which comprises a solid polymer electrolyte orion-exchange membrane disposed between the two electrode layers.Separator plates, or flow field plates. for directing the reactantsacross one surface of each electrode substrate, are disposed on eachside of the MEA. Solid polymer fuel cells operate at relatively lowtemperatures (circa 80° C.) compared to other fuel cell types.

A broad range of reactants can be used in electrochemical fuel cells.For example, the fuel stream may be substantially pure hydrogen gas, agaseous hydrogen-containing reformate stream, or methanol in a directmethanol fuel cell. The oxidant may be, for example, substantially pureoxygen or a dilute oxygen stream such as air.

The fuel stream may contain impurities that do not contribute to, andmay actually inhibit, the desired electrochemical reaction. Theseimpurities may, for example, originate from the fuel stream supplyitself, or may be generated, for example, as intermediate species duringthe fuel cell reactions, or may be impurities entering the fuel streamfrom elsewhere in the system. Some of these impurities may be chemicallyadsorbed or physically deposited on the surface of the anodeelectrocatalyst, blocking the active electrocatalyst sites andpreventing these portions of the anode electrocatalyst from inducing thedesired electrochemical fuel oxidation reaction. Such impurities areknown as electrocatalyst “poisons” and their effect on electrochemicalfuel cells is known as “electrocatalyst poisoning”. Electrocatalystpoisoning thus results in reduced fuel cell performance, where fuel cellperformance is defined as the voltage output from the cell for a givencurrent density. Higher performance is associated with higher voltagefor a given current density or higher current for a given voltage.

In the absence of countermeasures, the adsorption or deposition ofelectrocatalyst poisons may be cumulative, so even minute concentrationsof poisons in a fuel stream, may, over time, result in a degree ofelectrocatalyst poisoning which is detrimental to fuel cell performance.

Reformate streams derived from hydrocarbons or oxygenated hydrocarbonstypically contain a high concentration of hydrogen fuel, but typicallyalso contain electrocatalyst poisons such as carbon monoxide. To reducethe effects of anode electrocatalyst poisoning, it is known to pre-treatthe fuel supply stream prior to directing it to the fuel cell. Forexample, pre-treatment methods may employ catalytic or other means toconvert carbon monoxide to carbon dioxide. However, known pre-treatmentmeans for reformate streams cannot efficiently remove 100% of the carbonmonoxide. Even trace amounts less than 10 parts per million (ppm can)eventually result in electrocatalyst poisoning which causes a reductionin fuel cell performance.

Substances other than carbon monoxide are also known to poison fuel cellelectrocatalysts. Poisons may also be generated by the reaction ofsubstances in the reactant streams with the fuel cell componentmaterials. For instance, carbon monoxide or other impurities can begenerated from carbon dioxide in the presence of an electrocatalyst.This can occur when there is a relative abundance of carbon dioxide anda relatively low concentration of carbon monoxide such that equilibriumconditions favor some limited carbon monoxide formation. Whatconstitutes a poison may depend on the nature of the fuel cell. Forexample, whereas methanol is the fuel in a direct methanol fuel cell, ina hydrogen fuel cell operating on a methanol reformate stream, traces ofunreformed methanol can be detrimental to the electrocatalystperformance.

Conventional methods for addressing the problem of anode electrocatalystpoisoning include purging the anode with an inert gas such as nitrogen.However, such purging methods involve suspending the generation of powerby the fuel cell, thus a secondary power source may be needed to providepower while the fuel cell anode is being purged.

Another approach for removing poisons from an electrocatalyst comprisesintroducing a “clean” fuel stream containing substantially no carbonmonoxide or other poisons to a poisoned fuel cell anode. Where theadsorption is reversible, an equilibrium process may result in somerejuvenation of the electrocatalyst. However, a disadvantage of thisapproach is that it is generally not effective against irreversiblyadsorbed poisons. Furthermore, the recovery of the anode electrocatalystby such an equilibrium process can be very slow, during which time thefuel cell is not able to operate at full capacity.

Another technique for counteract carbon monoxide electrocatalystpoisoning is to continuously introduce a low concentration of oxygeninto the fuel stream upstream of the fuel cell, as disclosed in U.S.Pat. No. 4,910,099. Therein, oxygen levels from about 2% to 6% wereinjected into fuel streams having carbon monoxide levels from about 100to 500 ppm. However, such an oxygen bleed into the fuel stream resultsin some consumption of hydrogen and hence a reduction in fuelefficiency. Further, an oxygen bleed results in undesirable localizedexothermic reactions at the anode, particularly near the fuel inlet,which may damage fuel cell membranes and reduce fuel cell lifetime.Further still, since in practice oxygen bleed is typically obtained bycompressing air (it is often drawn from a compressed air stream providedas a fuel cell oxidant), use of an oxygen bleed may result in anadditional parasitic load on a fuel cell system. Thus, the use of anexcessive amount of oxygen bleed is undesirable.

To efficiently and effectively counteract carbon monoxide poisoning, itis desirable to know the approximate concentration of carbon monoxide inthe fuel stream. However, directly measuring the concentration of carbonmonoxide in a fuel stream can be difficult in practice. Thus, while asensor for directly measuring carbon monoxide is desirable in a fuelcell system, often the concentration is inferred based on the knownoperating conditions of the fuel cell system (for example, carbonmonoxide concentrations can be determined under various operatingconditions in a laboratory and can then be inferred for a fuel cellsystem operating under similar conditions in actual use). While manyearly fuel cell applications may have had relatively constant operatingconditions and therefore relatively consistent levels of carbon monoxidein the fuel stream, the potential applications for fuel cells areexpanding. As a result, the carbon monoxide concentration in the fuelstreams now tends to vary for many reasons (for example, depending onreformer temperature, or fuel cell and/or reformer load demandconditions).

A low output voltage from one or more of the fuel cells in a fuel cellsystem might be used as an indicator of carbon monoxide poisoning.Preferably perhaps, a sensor cell might be incorporated in the fuel cellsystem for this purpose. As described in U.S. Provisional PatentApplication Ser. No. 60/091,531 filed Jul. 2, 1998, by the sameapplicant as the present application and previously incorporated hereinby reference in its entirety, a sensor cell can be incorporated whoseperformance is more sensitive to carbon monoxide poisoning than otherfuel cells in the fuel cell system. Thus, a sensor cell can be used toindicate an abnormal or undesirable operating condition (for example,high level of CO) before it affects the performance of the other fuelcells.

U.S. patent application Ser. No. 08/998,133, filed Dec. 23, 1997, by thesame applicant as the present application, discloses a fuel celloperating method in which a substantially fuel-free liquid (which maycontain oxygen) is periodically introduced into the fuel stream in orderto cause a fuel starved condition.

SUMMARY OF THE INVENTION

Introducing a variable concentration of oxygen into an impure fuelstream can be advantageous for the removal of fuel stream impurities. Ifoxygen is introduced in accordance with the impurity level in the fuelstream, fuel losses, localized heating, and parasitic losses can bereduced. Further, a method involving a periodic or intermittentintroduction of oxygen has been shown to be effective to removeimpurities using a smaller integrated amount of oxygen than a methodinvolving a constant introduction of oxygen.

These advantages may be obtained in a fuel cell system which includes afuel cell operating on a stream of impure fuel supplied to the fuelcell, and which includes a mechanism for introducing oxygen into thefuel stream for reaction with an impurity in the fuel stream within thefuel cell. The method is particularly suitable for use in systemscomprising solid polymer fuel cells that operate at relatively lowtemperatures.

It can be advantageous to vary the concentration of oxygen introduced inaccordance with a fuel cell system operating characteristic indicativeof the concentration of the impurity. Suitable operating-characteristicsfor this purpose include the voltage of a sensor fuel cell incorporatedin the system that is sensitive to a particular impurity, or theconcentration of an impurity monitored or measured by an impurity sensorlocated somewhere in the fuel stream. Additionally, other suitableoperating characteristics include the voltage of another fuel cell inthe system, the voltage drop across a part of a cell (for example, thevoltage differential between parts of a cell associated with a partiallength of a flow field), and the temperature of a component of the fuelcell system (for example, the temperature of a reformer). Theconcentration of oxygen introduced into the fuel stream is desirablyadjusted in response to measured or monitored variations in one or moreof these operating characteristics.

In principle, it can also be advantageous to vary the amount of oxygenintroduced into the fuel stream independently of the operating state ofthe fuel cell system. For instance, it can be advantageous to introduceoxygen into the fuel stream periodically, such as in a series ofperiodic pulses rather than in a continuous (steady state) manner. Thistechnique results in a similar removal of impurities, but using asmaller total amount of oxygen. It is particularly preferred to alsovary the concentration of oxygen introduced in accordance with a fuelcell system operating characteristic indicative of the concentration ofthe impurity.

A periodic or cyclic variation in the concentration of oxygen introducedinto the fuel stream can be characterized by a waveform (for example,sinusoidal, sawtooth, square wave pulse). Further, the concentration ofoxygen introduced can be varied cyclically but also in proportion to anoperating characteristic of the fuel cell system indicative of impurityconcentration, such as in a series of pulses whose number or amplitudevary in proportion to an impurity concentration detected.

The method is particularly suitable for substantially reducing theeffect of or preventing carbon monoxide poisoning of an anodeelectrocatalyst, although similar benefits can be expected with regardsto poisons originating from carbon dioxide or other oxidizableimpurities. The method is effective when the fuel stream comprises atleast up to about 1000 ppm carbon monoxide.

In the preceding, oxygen introduced may be substantially pure oxygen, ina dilute oxygen stream such as air, in an oxygen containing solution, orgenerated in-situ from a suitable compound such as hydrogen peroxide. Ina preferred embodiment of the method, for use with fuel streamscomprising carbon monoxide impurity, less than about 4% air by volume isintroduced in periodic pulses. The period of the pulses may be in therange from about 1 to 50 seconds. The pulse duration or width may beless than about half the period of the pulses. In addition to suchperiodic pulses of air, a steady baseline concentration of air bleed(for instance less than about 0.8% by volume of the fuel stream) may bemaintained in the fuel stream between pulses. In this way, some lowerlevel of oxygen bleed is maintained at all times for baseline carbonmonoxide scavenging or for purposes of carbon dioxide scavenging.

Embodiments of the methods described above can substantially reduce theeffect of electrocatalyst poisoning without substantially affecting theelectrical output of the fuel cells in the system. As long as asufficient concentration of fuel is maintained in the fuel stream,temporary fuel starvation can be generally avoided. Fuel starvationoccurs when the fuel stoichiometry (the ratio of the amount of fuelsupplied to the amount of fuel actually consumed in the electrochemicalreactions) is less than 1 and is characterized by a rise in the anodevoltage in the system fuel cells, possibly resulting in fuel cellreversal in some cells (that is, where the cell voltage goes belowzero).

To carry out embodiments of the above method, a mechanism is included inthe fuel cell system for introducing a variable amount of oxygen intothe fuel stream for reaction with an impurity in the fuel stream withinthe fuel cell. The oxygen introducing mechanism can comprise, forexample, a flow controller for periodically introducing pulses of oxygeninto the fuel stream. Further, the system can comprise one or moremonitoring devices for monitoring a fuel cell operating characteristic,such as those described above, and a controller responsive to themonitoring device for adjusting the concentration of oxygen introducedinto the fuel stream. The monitoring device may preferably comprise asensor fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a solid polymer fuel cell systemcomprising a reformers a variable air bleed flow controller, and acarbon monoxide sensor cell.

FIG. 2 is a plot showing the voltage of a representative fuel cell and asensor cell versus time when 200 ppm carbon monoxide is introduced intothe fuel stream.

FIGS. 3a and 3 b are plots showing the voltage of a representative fuelcell and a sensor cell versus time at different current densities andexposed to a fuel stream with varied carbon monoxide levels toillustrate how the sensor cell may be used to trigger a variable airbleed.

FIG. 3c is a plot showing the voltage of a representative fuel cell anda sensor cell versus time in a system in which the sensor cell is usedto trigger the introduction of a series of air bleed pulses into thefuel stream in response to a transient increase in carbon monoxidelevel.

FIG. 4 is a plot showing the voltage of a representative fuel cell and asensor cell versus time in a system in which the sensor cell is used totrigger the introduction of a series of air bleed pulses in response toa carbon monoxide level of 100 ppm.

FIG. 5 is a plot showing the voltage of a representative fuel cellversus average air bleed concentration in which the air bleed was eitherpulsed or constant.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

A preferred embodiment of a fuel cell system equipped to introduce avariable amount of oxygen into the fuel stream directed to the fuelcells in the system is shown in the schematic diagram of FIG. 1. Thesystem comprises a solid polymer fuel cell stack 1 and a fuel processor2 comprising a reformer. A supply of fuel 3 (for example, methanol)provides feedstock to the reformer. The fuel is reformed and processed,generating a hydrogen rich but impure fuel stream comprising asignificant concentration of carbon monoxide impurity. The impure fuelstream is supplied to the fuel inlet 5 of the fuel cell stack 1. Asupply of oxidant 4 (for example, compressed air) is supplied to theoxidant inlet 6 of the fuel cell stack 1. As depicted, the fuel andoxidant streams supplied to the fuel cell stack 1 are exhausted at fueland oxidant outlets 7 and 8 respectively. The system also comprises anoxygen supply and a flow controller 10 for introducing a variable amountof oxygen into the fuel stream upstream of the fuel cell stack at 11.For simplicity, the oxygen supply is preferably air, obtained via ableed line 9 branching off the oxidant stream supplied to the fuel cellstack 1. As depicted, the system also comprises a sensor cell 12 whosevoltage output or performance is particularly sensitive to carbonmonoxide. Sensor cell 12 is preferably situated within the fuel cellstack 1. Thus, sensor cell 12 effectively monitors the carbon monoxidelevel in the fuel stream directed to stack 1. The voltage of the sensorcell 12 is then used as a signal to adjust flow controller 10.

Flow controller 10 controls the concentration of air introduced into thefuel stream. Flow controller 10 comprises a flow control valve having anopening that is variable in accordance with the signal from sensor cell12 and with the desired bleed method (for example, pulsed or otherwaveform, proportional, or combinations thereof). For example, the ratioof the sensor cell voltage to the average voltage of fuel cells in thestack may be used to trigger an air bleed pulse. The timing of an airbleed pulse and the concentration of air introduced will depend on thecharacteristics and operation of the sensor cell and the fuel cellstack. Also, depending on the operation of the fuel cell system, it maybe necessary to monitor certain other system parameters to achieve thiscontrol. For example, if the fuel stream flow rate is variable (which ittypically is), then the amount of air to be introduced will also be afunction of that, if a particular concentration is to be maintained.Thus, the fuel stream flow rate may need to be monitored and used as aninput to flow controller 10. Flow controller 10 may comprise a processorfor processing multiple input signals to determine a desired setting fora flow control valve. Those skilled in the art may readily determine asuitable configuration of conventional flow controllers and/or parametermeasuring devices for a specific fuel cell system application.

In this embodiment then, flow controller 10 can be used to adjust theamount of air bleed in accordance with the concentration of carbonmonoxide entering the fuel cell stack 1. In systems where the carbonmonoxide level varies significantly, the air bleed can thus be adjustedto reduce or eliminate the effect of carbon monoxide poisoning withoutusing an excess of introduced air. The ability to adjust air bleed isparticularly beneficial during start-up of many reformers where higherlevels of carbon monoxide are typically produced, or during rapidlyvarying fuel cell loads where the reformer output needs to respond totransient load increases and the quality of the fuel stream may vary.

In addition, flow controller 10 can be used to introduce air in aperiodic or cyclic fashion. For example, a programmable flow controllercan be programmed to admit air according to some programmed waveform(for example, sinusoidal, sawtooth, square wave). Alternatively, a flowcontroller that oscillates between two or more different states (forexample open/closed) as determined by some measured input caneffectively introduce air in periodic fashion (as in the periodic airpulses shown in FIG. 4 following). This feature is beneficial sinceenhanced reactivity of impurities can occur on electrocatalysts duringperiodic operation as opposed to steady state operation. Particularly,the reaction of carbon monoxide and oxygen can be enhanced on platinumtype electrocatalysts. By introducing air in a periodic fashion, it ispossible to mitigate the effect of carbon monoxide poisoning using lessoxygen than would otherwise be required when introducing a constantconcentration of air. Therefore, while the system in FIG. 1 represents apreferred embodiment, a benefit can be obtained even when the flowcontroller 10 operates independently of any system characteristic (forexample, independent of sensor cell 12). With a periodic introduction ofair, less oxygen needs to be introduced to mitigate the effect of carbonmonoxide poisoning to the same extent. Thus, such a system can alsoachieve improved performance over that obtained with a constant airbleed.

If the air bleed is introduced independently of the concentration ofimpurity present, a suitable pulsed air bleed system can be quite simplein principle. By way of example, the flow controller 10 in FIG. 1 mightcomprise a check valve in a system employing flow reversal of theoxidant stream (as described in U.S. patent application Ser. No.08/980,496, filed Dec. 1, 1997, by the same applicant as the presentapplication). With oxidant flow reversal, the oxidant flow directionthrough the fuel cells reverses periodically (that is, inlet 6 andoutlet 8 periodically become outlet 6 and inlet 8 in FIG. 1) and theline supplying inlet 6 undergoes a high and low pressure cycle with eachreversal. The check valve and bleed line geometry 9 could be set to givea specific oxidant bleed into the fuel line during the high pressurepart of the cycle in the line supplying inlet 6 and to give no oxidantbleed during the low pressure part of the cycle. In this example, it maybe desirable to incorporate means for controlling the amount of oxygenintroduced during the high pressure part of the cycle.

Nonetheless, a particularly preferred air bleed method involves theperiodic introduction of air in accordance with a measurement indicativeof the concentration of impurity present. The use of a series of pulsesis a convenient way of introducing a given amount of air in a periodicfashion. The amount of air admitted to the fuel stream at any given timeis determined by the amplitude of a pulse. The total amount of airadmitted over a given time period however is determined by theamplitude, width, and frequency of pulses. The preferred set of pulsecharacteristics can be expected to vary somewhat depending on the fuelcell system. For instance, depending on the reaction kinetics and heattransfer characteristics in specific fuel cells, it may be advantageousto employ pulse amplitudes that are greater than conventional air bleedlevels without making localized heating significantly worse.Nonetheless, when the pulse amplitude is less than or equal to constantconventional air bleed levels, temporary localized temperature increasesmay be reduced and temporary fuel starvation in the fuel cell stack maybe avoided. Conversely however, the pulse amplitude is preferably not sosmall that oxidation of the impurity is not complete.

The following examples have been included to illustrate differentembodiments and aspects of the invention but these should not beconstrued as limiting in any way. For instance, while the variableoxygen introduction method is particularly advantageous for use insubstantially reducing the effect of or preventing carbon monoxidepoisoning in solid polymer fuel cells, the method may also be effectivewith other impurities, such as carbon dioxide or methanol, in these orother types of fuel cells.

EXAMPLES

A fuel cell system for test purposes was configured generally asoutlined in FIG. 1 and the preceding description. The fuel cell stackconsisted of a number of conventional solid polymer fuel cells and asensor cell. The membrane electrode assemblies in the conventional cellsemployed a cathode and anode having platinum and platinum-rutheniumelectrocatalysts, respectively, applied to porous carbon fiber papersubstrates (TGP-090 grade from Toray), and a Nafion™ solid polymermembrane. The sensor cell was constructed to be sensitive to carbonmonoxide as described in the aforementioned U.S. Provisional PatentApplication Ser. No. 60/091,531, and differs from the conventional fuelcells primarily in the choice of electrocatalyst and the ionomer loadingused in the anode. The sensor cell employed a platinum blackelectrocatalyst loaded at about 1 mg/cm² and about a 40% ionomer loading(approximately twice that of the conventional fuel cells).

The fuel supply used was a humidified gas comprising about 63% hydrogen,20% carbon dioxide, 17% nitrogen and varied concentrations of carbonmonoxide impurity as indicated in the following. Fuel was supplied at apressure of about 23 psig and with a stoichiometry (that is, the ratioof the amount of fuel supplied to the amount of fuel actually consumedin the electrochemical reactions) of about 1.2. The oxidant used was airat a pressure of about 23 psig and with a stoichiometry of about 1.5.The fuel cell stack was operated at approximately 75° C. and at currentdensities as indicated below.

FIG. 2 shows the voltage of a representative conventional fuel cell(“FC”) and the sensor cell (“SC”) versus time at a current density of500 mA/cm² when 200 PPM carbon monoxide was introduced into the fuelstream. Also shown in this FIG. is the carbon monoxide level asdetermined by an infrared carbon monoxide sensor located upstream of thehumidifier in the fuel inlet of the stack. No air bleed was used in thiscomparative example. Initially (time=0), the voltages of therepresentative fuel and sensor cells are about 0.7 and 0.6 voltsrespectively. With the introduction of carbon monoxide in the fuelstream, the voltage of both cells dropped substantially, although thesensor cell reacted much more quickly and adversely to the presence ofcarbon monoxide. This example shows the adverse effect of carbonmonoxide on fuel cell performance and also demonstrates theeffectiveness of the sensor cell in early detection of carbon monoxidecontamination.

The performance was then restored by introducing 4% air (by volume) intothe fuel stream to oxidize carbon monoxide in the cells. The flowcontroller was then set so as to introduce a baseline level of 0.8% airinto the fuel stream that would increase to a 4% air bleed level if thesensor cell voltage fell more than 100 mV. FIG. 3a shows voltages of therepresentative fuel cell, FC, and the sensor cell, SC, versus time (t)at a current density of 500 mA/cm². (In FIG. 3a, the time axis origin,t=0, is located 25 seconds after data collection starts.) The carbonmonoxide concentration is also shown. Initially, a baseline level of 2ppm carbon monoxide was introduced into the fuel stream. The initialvoltages of each cell were thus not quite the same as those in FIG. 2due to the presence of carbon monoxide and air bleed. At t=0 seconds,the carbon monoxide level was increased to approximately 100 ppm. Atabout t=30 seconds, the sensor cell voltage dropped sufficiently totrigger the flow controller, thereby increasing the air bleed level to4%. The sensor cell voltage can be seen to recover somewhat thereafter.The voltage of the representative fuel cell however is essentiallystable over this whole period. Thus, using the sensor cell, it ispossible to actually prevent carbon monoxide from adversely affectingperformance of the conventional cells in the system.

FIG. 3b shows the results of a similar test except the cells wereoperated at a current density of 100 mA/cm² and the carbon monoxidelevel was increased to 400 ppm (instead of 100 ppm). The plot shown inFIG. 3b begins (that is t=0) at the point when the carbon monoxide levelwas increased from 2 to 400 ppm carbon monoxide. Again, the sensor celltriggers at about t=44 seconds, raising the air bleed level from 0.8% to4%, and thereby maintaining the voltage of the representative fuel cell.

FIG. 3c shows the results of a similar test except the cells wereoperated at a current density of 500 mA/cm² and the carbon monoxidelevel was first increased to 40 ppm at t=0 seconds and then decreasedagain to 2 ppm at about t=635 seconds. (In FIG. 3c, the time axisorigin, t=0, is also located 25 seconds after data collection starts.)The state of the flow controller (that is air bleed level) versus timeis also shown schematically in FIG. 3c. Under these conditions, thesensor cell voltage first dropped over 100 mV thereby triggering anincrease in air bleed. However, the sensor cell voltage then recoveredafter 30 seconds thereby triggering a return to the baseline air bleedlevel. Eventually, about 40 seconds later, the sensor cell voltage againdropped to trigger the increased air bleed level and these pulsescontinued until the carbon monoxide concentration was reduced from 40ppm back to 2 ppm. The net result of this oscillation is that the airbleed was supplied in a pulsed, periodic manner during the period ofhigh carbon monoxide concentration in the stream. Again, the voltage ofthe representative fuel cell was maintained throughout. These resultsshow that a variable air bleed method, which uses the output of a sensorcell for control, can be effective in practice to deal with varyinglevels of carbon monoxide in the fuel stream of an operating fuel cell.

Another test was performed as above except that the flow controller wasset so as to introduce a baseline level of 0% air (none) into the fuelstream and to increase it to a 4% air bleed level if the sensor cellvoltage fell below 350 mV. FIG. 4 shows voltages of the representativefuel cell and the sensor cell versus time (t) at an operating currentdensity of 500 mA/cm² with a substantially constant 100 ppm level ofcarbon monoxide in the fuel stream over the period shown. Also shownschematically is the state of the flow controller versus time. As inFIG. 3c, the sensor cell voltage and the state of the flow controlleroscillate, resulting in the introduction of a series of pulses of airinto the fuel stream. Throughout, the voltage of the representative fuelcell remains essentially constant. The pulse width or duration of theair bleed was slightly less than half the oscillation period. Since airis approximately 20% oxygen, the pulsed air bleed in FIG. 4 was roughlyequivalent to the continuous introduction of about 0.4% oxygen over thetime period shown. This appears to prevent poisoning in conventionalfuel cells operating on fuel streams containing at least up to 100 ppmcarbon monoxide. However, in the aforementioned U.S. Pat. No. 4,910,099,it appears that over 2% oxygen is required to prevent poisoning from 100ppm carbon monoxide.

Finally, a test was performed to compare fuel cell performance versusair bleed concentration when an air bleed was introduced either in aseries of pulses or at a conventional constant concentration. A fuelcell stack similar to the preceding stack was used, but without a sensorcell. The fuel cell stack was operated as above at a current density of500 mA/cm² and with a constant 40 ppm carbon monoxide impurity in thefuel stream. Initially, no air bleed was employed. The system wasallowed to run for about 15 minutes and the voltage of a representativefuel cell in the stack was recorded. Air was then introduced in squarewave pulses with a pulse duration of 5 seconds and a period of 10seconds (that is, 5 seconds on, 5 seconds off). No air was introducedbetween pulses. The air bleed concentration was increased in discretesteps, and each time after running for about 15 minutes, the voltage ofthe representative fuel cell was again recorded. The air bleed was thenstopped and the test was repeated using constant concentrations of airbleed. FIG. 5 shows a plot of the voltage of the representative fuelcell versus average air bleed % with pulsed air bleed (“P”) and withconstant air bleed (“C). (In this example, the average % air bleedconcentration in the pulsed trial is thus approximately half theconcentration during a pulse.) As can readily be seen in FIG. 5, lessair is required to obtain a given cell voltage (that is, a givenperformance) using pulsed rather than constant air bleed.

While particular elements, embodiments and applications of the presentinvention have been shown and described, it will be understood, ofcourse, that the invention is not limited thereto since modificationsmay be made by those skilled in the art without departing from the scopeof the present disclosure, particularly in light of the foregoingteachings.

What is claimed is:
 1. A method of operating a fuel cell system on animpure fuel, said system including at least one fuel cell, a stream ofsaid impure fuel supplied to said fuel cell, and a mechanism forintroducing oxygen into said fuel stream for reaction with an impurityin said fuel stream within said fuel cell, said method comprisingintroducing a variable concentration of oxygen into said fuel streamperiodically.
 2. The method of claim 1 wherein said oxygen is introducedinto said impure fuel stream in a series of periodic pulses.
 3. Themethod of claim 1 wherein said impurity comprises carbon monoxide. 4.The method of claim 3 wherein said fuel stream comprises up to about1000 ppm carbon monoxide.
 5. The method of claim 1 wherein said impuritycomprises carbon dioxide.
 6. The method of claim 2 wherein the period ofthe pulses is in the range from about 1 to 50 seconds.
 7. The method ofclaim 2 wherein said oxygen is introduced into said impure fuel streamin a series of periodic pulses of air.
 8. The method of claim 7 whereinthe concentration of air introduced into said fuel stream during saidperiodic pulses is less than about 4% by volume of said fuel stream. 9.The method of claim 7 wherein a substantially continuous baselineconcentration of air is introduced into said fuel stream between saidperiodic pulses of air.
 10. The method of claim 9 wherein said baselineconcentration of air is less than about 0.8% by volume of said fuelstream.
 11. The method of claim 2 wherein the duration of said periodicpulses is less than about half the period.
 12. The method of claim 1wherein said at least one fuel cell is a solid polymer fuel cell. 13.The method of claim 1 further comprising maintaining the fuelstoichiometry in said fuel cell at greater than or equal to
 1. 14. Afuel cell system comprising: at least one fuel cell; a source of animpure fuel stream for supply to said fuel cell; a mechanism forintroducing a variable concentration of oxygen into said impure fuelstream for reaction with an impurity in said fuel stream within saidfuel cell; and a monitoring device for monitoring a fuel cell operatingcharacteristic indicative of the concentration of said impurity in saidfuel stream; and a flow controller responsive to the monitoring devicefor adjusting said oxygen concentration in response to said monitoredoperating characteristic.
 15. The fuel cell system of claim 14 whereinsaid monitoring device comprises a sensor fuel cell.
 16. The fuel cellsystem of claim 14 further comprising a mechanism for maintaining asufficient amount of fuel in said impure fuel stream such that the fuelstoichiometry is greater than or equal to
 1. 17. A fuel cell systemcomprising: at least one fuel cell; a source of an impure fuel streamfor supply to said fuel cell; and a mechanism for introducing a variableconcentration of oxygen into said impure fuel stream for reaction withan impurity in said fuel stream within said fuel cell wherein saidoxygen introducing mechanism comprises a flow controller forperiodically introducing oxygen into said fuel stream.